This invention relates generally to methods and apparatus for balancing of relatively slowly rotating objects having large open bores with no central shafts, and more particularly to methods and apparatus for balancing of such rotating equipment as is found in computed tomography (CT) imaging systems.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The rotational speed of gantries of CT scanners has continued to increase, until CT scanners now produce rotational speeds of 120 revolutions per minute (RPM). Speeds are expected to continue to increase. However, as rotational speeds increase, so does the need to dynamically balance the rotating portion of the scanner.
The balancing of rotational equipment is not a new concept. Balancing technology is well-understood, and several companies produce commercial balancing equipment. However, because of the nature of CT imaging system design, there are a number of reasons why traditional balancing equipment will not work with a CT system. Some of the more significant reasons include the following:
Most rotating equipment that is balanced by commercial equipment has a solid central shaft; for example, electric motors and fans. CT equipment, on the other hand, has a large, open bore, typically about 700 mm or larger.
Most rotating equipment has two bearings positioned at opposite ends of the rotating member. By contrast, the main bearing member of a CT imaging system has either one bearing, or two bearings that are positioned very close to each other.
Most commercially available balancing equipment requires rotational speeds of 100 RPM or more to accurate balance a rotating system, as most equipment requiring balancing rotates as 1000 RPM or higher. CT imaging systems not having the highest rotational speed capability do not reach the minimum rotational speed required by the balancing equipment, and systems having the highest rotational speeds barely reach the minimum.
Also, typical rotating equipment usually has its center of gravity (CG) positioned between its two support bearings, and rarely has much of its rotating load cantilevered. By contrast, most CT imaging systems have an over-hung load, relative to the system""s main bearing plane.
FIG. 3 illustrates one method by which conventional balancing is performed. An axis 50 of a rotating member 52 is positioned horizontally, with the center of gravity CG placed between two elastic mounts 54, 56 of a balancing machine 58. A coupling 60 of balancing machine 58 is rotatably coupled to a shaft 62 of rotating member 52. A drive mechanism 64 of balancing machine 58 rotates coupling 60 which causes rotating member 52 to rotate. A nonsymmetrical distribution of weight around axis 50 of rotating member 52 results in translational motion of both elastic mounts 54, 56. Balancing machine 58 measures translational motion of both elastic mounts 52, 54 via accelerometers 65, 66, as well as the relative phasing of their motion. Adjustments are made (if any are necessary) in the distribution of weight of rotating member 52 to reduce the translational motion, the redistribution being a function of the relative phasing of motion. Once satisfactorily balanced, rotating member 52 goes through a final assembly and is usually never subsequently modified, serviced, or replaced.
With CT imaging systems, there are at least two problems with the above approach. First, the rotating member of a CT imaging system is comprised of several discrete components, some mechanical and some electrical. Throughout the life of the CT imaging system, any one of these components may require modification or replacement, potentially unbalancing the system.
Also, because the rotating mass of a CT imaging system is overhung, there is no xe2x80x9csecondxe2x80x99 mount available to measure the motion and the angular phasing of the imbalance of the rotating mass. The frame of the CT gantry cannot be used as a secondary mount for measuring the motion of the rotating mass due to the confounding of motion information from the natural resonant motion of the frame itself (i.e., the accelerometer senses motion from both the rotating mass and the frame, whereas dynamic balancing requires just the motion of the rotating mass).
It would therefore be desirable to provide methods and apparatus for balancing a CT imaging system both at the end of a manufacturing cycle as well as in the field after component replacement.
One aspect of the present invention is therefore a method for balancing a hollow cylindrical, rotatable object that is coupled to a drive source that is configured to rotate the rotatable object without a central shaft. The method includes steps of: mounting an arbor having a forward shaft and an aft shaft to an inner wall of the rotatable object so that the forward shaft and aft shaft are concentric to an axis of rotation of the rotatable object; operating the drive source to rotate the rotatable object and the arbor; measuring displacement of the forward shaft and the aft shaft of the arbor while the drive source is rotating the rotatable object; applying balancing weights to the rotatable object in accordance with the measured displacements; and removing the arbor from the rotatable object when a balance is achieved.
The above-described method for balancing a rotatable object is suitable for balancing CT imaging systems both at the end of a manufacturing cycle as well as in the field after component replacement. In addition, because the rotatable object is balanced using an integral system, it is not necessary to uncouple the rotatable object from the frame of the CT gantry.